Rsync to /dev/null overwrites it

I am seeing a random behavior when copying from one server to another using rsync to the /dev/null of the destination server. This is as part my network performance testing, and I want to avoid Disk I/O in my testings. I have the same script in all my servers, but for some of them the destination /dev/null gets overwritten and becomes a regular file, and that breaks other things.